A variety of actuators are used in downhole well systems and other types of systems to actuate tools between operational positions. The actuator is coupled to a movable element of a corresponding tool to enable controlled shifting of the tool between the operational positions. In many downhole, well applications, for example, hydraulic actuators are employed and comprise a movable piston which can be shifted by applying suitable hydraulic pressure. Plastic and/or elastomeric materials are used to form a seal between the piston and a surrounding cylindrical wall of the actuator, but such materials may be susceptible to degradation in certain high temperature or otherwise deleterious downhole environments.